Planar Polarization Rotator

ABSTRACT

An optical polarization rotator includes first and second optical waveguide ribs located along a planar surface of a substrate. The second optical waveguide rib is located farther from the surface than the first optical waveguide rib. First segments of the optical waveguide ribs form a vertical stack over the substrate, and second segments of the optical waveguide ribs are offset laterally in a direction along the planar surface. The first and second optical waveguide ribs are formed of materials with different bulk refractive indexes.

BACKGROUND

1. Technical Field

The invention relates to polarization rotators and methods or making andusing polarization rotators.

2. Related Art

This section introduces aspects that may be helpful to facilitating abetter understanding of the inventions. Accordingly, the statements ofthis section are to be read in this light and are not to be understoodas admissions about what is in the prior art or what is not in the priorart.

Some optical components process one or both orthogonal linearpolarization component(s) separately to perform function(s) associatedwith the optical communication of a digital data stream. To enable suchprocessing, a polarization splitter may process received light toseparate the two orthogonal linear polarization components thereof.Additionally, to enable such processing, a polarization rotator mayrotate one or both of the separated linear polarization components ofsuch light. For example, such optical rotating may align thepolarizations of both of the separated polarization components.

SUMMARY

One embodiment provides an apparatus that includes an opticalpolarization rotator having first and second optical waveguide ribslocated along a planar surface of a substrate. The second opticalwaveguide rib is located farther from the surface than the first opticalwaveguide rib. First segments of the two optical waveguide ribs form avertical stack over the substrate. Second segments of the two opticalwaveguide ribs are offset laterally in a direction along the planarsurface. The first and second optical waveguide ribs are formed ofmaterials with different bulk refractive indexes.

In some embodiments, the above apparatus may further include a spacerlayer between the first and second optical waveguide ribs. Such a spacerlayer may be, e.g., formed of a material with a different bulkrefractive index than the materials of the first and second opticalwaveguide ribs. Such a spacer layer may be thinner vertical to thesurface than the first optical waveguide rib.

In some embodiments of any of the above apparatus, the first opticalwaveguide rib may be formed of a same material as a portion of theplanar surface of the substrate.

In some embodiments of any of the above apparatus, one of the opticalwaveguide ribs may be formed of a semiconductor, and the other of theoptical waveguide ribs may be formed of a dielectric.

In some embodiments of any of the above apparatus, the opticalpolarization rotator may be configured to rotate a polarization ofreceived linearly polarized light by at least 45 degrees.

In some embodiments of any of the above apparatus, the apparatus mayinclude a transition region in which the first optical waveguide rib hasa width lateral to the surface that monotonically tapers from a largervalue at a boundary of an input planar optical waveguide to a smallervalue at the first segment of first optical waveguide rib. In some suchapparatus, the second optical waveguide rib has a width lateral to thesurface that monotonically tapers from a smaller value at an end thereofnearer to the input optical waveguide to a larger value at the firstsegment of second optical waveguide rib.

In some embodiments of any of the above apparatus, the apparatus mayinclude a transition region in which the first optical waveguide rib hasa width lateral to the surface that monotonically tapers from a smallervalue at the second segment of first optical waveguide rib to a largervalue at a boundary of an output planar optical waveguide.

In some embodiments of any of the above apparatus, the apparatus mayinclude a polarization beam splitter having a first optical outputconnected to transmit light to the first optical waveguide rib and asecond optical output connected to a first specific output opticalwaveguide that is located over the substrate lateral to the firstoptical waveguide rib. In some such embodiments, the apparatus mayfurther include a second specific output optical waveguide connected tothe first optical output of the polarization beam splitter via theoptical polarization rotator. The apparatus may be configured totransmit light of substantially the same linear polarization to the twospecific output optical waveguides.

In some embodiments of the above apparatus, the apparatus may include anoptical modulator having a 1×2 optical splitter, first and secondoptical waveguide arms, and a 2×1 optical combiner. Each opticalwaveguide arm connects a corresponding optical output of the 1×2 opticalsplitter to a corresponding optical input of the 2×1 optical combinerand includes an electro-optical modulator capable of modulating thephase and/or amplitude of light propagating there through responsive toelectrical signals received therein. In such embodiments, the opticalpolarization rotator is located in one of the optical waveguide arms.

Another embodiment provides a method. The method includes forming anoptical layer over a first optical waveguide rib located along a planarsurface of a substrate. The optical layer and first optical waveguiderib are formed of materials with different bulk refractive indexes. Themethod also includes etching the optical layer to form a second opticalwaveguide rib. First segments of the optical waveguide ribs form a stackvertically oriented with respect to the planar surface. Second segmentsof the optical waveguide ribs are relatively laterally offset along theplanar surface.

In some embodiments, the above method may further include forming aspacer layer over the first optical waveguide rib prior to forming theoptical layer. The spacer layer is formed of a material with a differentbulk refractive index than the materials of the optical layer and thefirst optical waveguide layer.

In some embodiments of any of the above methods, the first opticalwaveguide and a portion of the planar surface may be formed of the sameoptical material. In some such embodiments, the etching may be performedsuch that the optical material functions as a stopping layer during theetching.

In some embodiments of any of the above methods, one of the opticallayer and the first optical waveguide rib may be formed of asemiconductor, and the other of the optical layer and the first opticalwaveguide rib may be formed of a dielectric.

In some embodiments of any of the above methods, third and fourthsegments of the first optical waveguide rib may be located lateral tothe second optical waveguide rib and may be laterally wider than thefirst and second segments of the first optical waveguide rib. In suchembodiments, the first and second segments of the first opticalwaveguide rib are connected between the third and fourth segmentsthereof.

Another embodiment provides a second method that includes receiving, ata first end of a segment of a hybrid optical waveguide, linearlypolarized light from a planar waveguide; and propagating the lightthrough the segment of the hybrid optical waveguide to rotate a linearpolarization of the light. The segment of the hybrid optical waveguideincludes one segment in which a second optical waveguide rib isvertically located over a first optical waveguide rib and includesanother segment in which the second optical waveguide rib issubstantially laterally offset from a corresponding segment of the firstoptical waveguide rib. The first and second optical waveguide ribs areformed of materials having different bulk refractive indexes.

In some embodiments of the second method, one of the first and secondoptical waveguide ribs may be a semiconductor rib and the other of thefirst and second optical waveguide ribs may be a dielectric rib.

In any of the above embodiments of the second method, the segment of thehybrid optical waveguide may include a spacer layer being locatedbetween the first and second optical waveguide ribs and being made of amaterial with a different bulk refractive index than the materials ofthe optical waveguide ribs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are respective top and side views that illustrate anoptical polarization rotator;

FIGS. 2A, 2B, and 2C are cross-sectional views schematicallyillustrating the evolution of the light power distribution of a firstoptical propagating mode in the optical polarization rotator of FIGS.1A-1B at lateral planes A, B, and C, respectively, of FIGS. 1A-1B;

FIGS. 3A, 3B, and 3C are cross-sectional views schematicallyillustrating the evolution of the light power distribution of a second,relatively orthogonal, optical propagating mode in the opticalpolarization rotator of FIGS. 1A-1B at the same respective, lateralplanes A, B, and C of FIGS. 1A-1B;

FIGS. 4A, 4B, and 4C are respective top, side, and oblique viewsschematically illustrating a specific embodiment of the opticalpolarization rotator of FIGS. 1A-1B;

FIG. 5 is a flow chart schematically illustrating a method offabricating an optical polarization rotator, e.g., the opticalpolarization rotator schematically illustrated in FIGS. 4A-4C;

FIG. 6 is a flow chart schematically illustrating a method to rotatepolarizations of linear polarized light, e.g., in the opticalpolarization rotators schematically illustrated in FIGS. 1A-1B and4A-4C;

FIG. 7 is block diagram schematically illustrating an optical modulatorthat incorporates an optical polarization rotator, e.g., one of theoptical polarization rotators of FIGS. 1A-1B or 4A-4C, to produce apolarization multiplexed, data-modulated optical carrier; and

FIG. 8 is block diagram schematically illustrating an optical receiverthat is configured to demodulate data separately from two linearpolarization components of an data-modulated optical carrier that ispolarization multiplexed, e.g., the optical carrier produced by theoptical modulator schematically illustrated in FIG. 7.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Herein, an optical waveguide refers to an optical structure that causesreceived light to propagate along a predefined optical guidingdirection. An optical waveguide is different from an optical structurethat simply confines received light to propagate in a plane. Herein, anoptical waveguide may refer to an unclad optical waveguide or theoptical core of a clad optical waveguide. That is, the expressionoptical waveguide covers both types of structures.

FIGS. 1A and 1B illustrate an optical polarization rotator 10 formed asan optical waveguide. The optical waveguide has a first transitionregion 12, a polarization rotation region 14, and a second transitionregion 16. The first transition region 12 optically end-connects thepolarization rotation region 14 to an input optical waveguide 18, e.g.,a planar optical waveguide. The polarization rotation region 14 rotatesa linear polarization of light received in one or two substantiallylinearly polarization propagating modes. The second transition region 16optically end connects the polarization rotation region 14 to an outputoptical waveguide 20, e.g., a planar optical waveguide.

The first and second transition regions 12, 16 are optical couplingregions with the respective input and output optical waveguides 18, 20.In some embodiments, the transition regions 12, 16 also redistributelight power density of propagating optical modes vertically or laterallywith respect to the nearby surface of the substrate 22, e.g., withoutrotating linear polarizations. The first transition region 12 mayinclude tapering of the optical waveguide that adiabatically mayredistribute said light power densities over larger vertical region. Thesecond transition region 16 may include tapering of the opticalwaveguide that adiabatically redistributes said received light powerdensities over smaller vertical region.

The polarization rotation region 14 includes an optical stack and asubstrate 22. The optical stack includes a first optical waveguide rib24 located on the substrate 22 and a second optical waveguide rib 26located farther from a planar surface of the substrate 22 than the firstoptical waveguide rib 24. The first and second optical waveguide ribs24, 26 are formed of materials having different bulk refractive indexes.For example, one of the optical waveguide ribs 24, 26 may be made of asemiconductor, and the other of the optical waveguide ribs 26, 24 may bemade of a dielectric. Alternatively, the first and second opticalwaveguide ribs 24, 26 may be made of different dielectrics or differentsemiconductors.

In some embodiments, the first optical waveguide rib 24 may be anintegral part of the substrate 22. Then, the first optical waveguide rib24 is formed of the same material as portions of the nearest planarsurface of the substrate 22.

Alternatively, the first optical waveguide rib 24 may be formed of adifferent material, e.g., of a material of different refractive index,than nearby portions the planar surface of the substrate 22.

Indeed, the first optical waveguide rib 24 may be located on and indirect contact with the nearby planar surface of the substrate 22 or maybe located over the nearby planar surface of the substrate 22 withoutbeing in direct contact with said planar surface.

The second optical waveguide rib 26 may be on and in direct contact withthe first optical waveguide rib 24 (not shown) or may be located overthe first optical waveguide rib 24 and separated from the first opticalwaveguide rib 24 by a spacer layer 28 of substantially opticallytransparent material. Such a spacer layer 28 is typically verticallythinner than the first optical waveguide rib 24.

In the polarization rotation region 14, the central axes of the firstand second optical waveguide ribs 24, 26 are oriented along diverginglateral directions. For that reason, the vertical overlap betweenlateral widths corresponding segments of the first and second opticalwaveguide ribs 24, 26 decreases along the direction of light propagationin the polarization rotation region 14, i.e., decreases to the right inFIGS. 1A-1B. Thus, some corresponding segments of the first and secondoptical waveguide ribs 24, 26 may be laterally offset along the planarsurface of the substrate 22, i.e., in right most portions of thepolarization rotation region 14.

In the polarization rotation region 14, each of optical waveguide ribs24, 26 may have a constant lateral width or may have a non-constantwidth. For example, the width of the first optical waveguide rib 24 mayincrease along the direction of light propagation in the polarizationrotation region 14. Also, the width of the second optical waveguide rib26 may decrease along the direction of light propagation in thepolarization rotation region 14.

The vertical widths of the first and second optical waveguide ribs 24,26 are typically substantially constant over the optical polarizationrotator 10.

FIGS. 2A, 2B, and 2C schematically illustrate expected light powerdistributions of a first propagating light mode at successive lateralplanes A, B, and C in the FIGS. 1A-1B. The first propagating mode isinitially a transverse magnetic (TM) propagating light mode, which hasan initial linear polarization.

At the initial lateral plane A, the lateral cross sections of the firstand second optical waveguide ribs 24, 26 are aligned vertically withrespect to the planar surface PS of the substrate 22. At the initiallateral plane A, the optical power of the first propagating light mode,i.e. a TM mode, is concentrated in a first oblong region OR1 whose majoraxis is vertically oriented with respect to the planar surface PS of thesubstrate 22.

At the later lateral plane B, centers of the cross sections of the firstand second optical waveguide ribs 24, 26 are substantially laterallyoffset, and the optical power of the first propagating light mode isconcentrated in a second oblong region OR2. The oblong region OR2 isstrongly tilted with respect to the normal direction to the planarsurface PS of the substrate 22 so that the oblong region's major axis isapproximately oriented along a diagonal between centers of the twooptical waveguide ribs 24, 26.

At the final lateral plane C, the cross sections of the first and secondoptical waveguide ribs 24, 26 are separated by a large lateral gap, andthe optical power of the first propagating light mode is concentrated ina third oblong region OR3 whose major axis is approximately parallel tothe planar surface of the substrate 22.

Thus, the slow evolution of the optical stack gradually rotates theoblong region in which the optical power of the first light propagatingmode is concentrated. During the gradual rotation, the linearpolarization of the first propagating light mode rotates with the oblongregion in which the optical power of the mode is concentrated. In theillustrated example, the polarization rotation is by about π/2 radiansthereby changing the initial TM propagating light mode into a finaltransverse electric (TE) propagating light mode.

FIGS. 3A, 3B, and 3C schematically illustrate expected light powerdistributions of a second propagating light mode at the same respectivelateral planes A, B, and C in the FIGS. 1A-1B. The first propagatingmode is initially a TE propagating light mode, which has an initiallinear polarization and is relatively orthogonal to the firstpropagating light mode.

At the initial lateral plane A, the cross sections of the first andsecond optical waveguide ribs 24, 26 are vertically aligned, and theoptical power of the second propagating light mode, i.e. a TE mode, isconcentrated in a different first oblong region OR1′. The first oblongregion OR1′ has a major axis that is horizontally oriented parallel tothe planar surface of the substrate 22.

At the later lateral plane B, the centers of the cross sections of thefirst and second optical waveguide ribs 24, 26 are substantiallylaterally offset, and the optical power of the second propagating lightmode is concentrated in a different second oblong region OR2′. Theoblong region OR2′ is strongly tilted with respect to the normaldirection to the planar surface of the substrate 22 so that the oblongregion's major axis is approximately oriented along a diagonal betweencenters of the two optical waveguide ribs 24, 26.

At the final lateral plane C, the cross sections of the first and secondoptical waveguide ribs 24, 26 are separated by a large lateral gap, andthe optical power of the second propagating light mode is concentratedin a different third oblong region OR3′ whose major axis isapproximately oriented perpendicular to the planar surface of thesubstrate 22.

Thus, the slow evolution of the optical stack also gradually rotates theoblong region in which the optical power of the second propagating lightmode is concentrated. During the gradual rotation, the linearpolarization of the second propagating light mode rotates with the majoraxis of the oblong region in which the optical power of the mode isconcentrated. In the illustrated example, the rotation is by about π/2radians thereby changing the initial TE propagating light mode into afinal TM propagating light mode.

FIGS. 4A, 4B, and 4C illustrate a specific example 10′ of the opticalwaveguide 10 illustrated in FIGS. 1A-1B. The optical waveguide 10′ hasthe first transition region 12, the polarization rotation region 14, andthe second transition region 16. In the polarization rotation region 14and a segment of the first transition region 12, the optical waveguide10′ includes the first and second optical waveguide ribs 24, 26. Thefirst optical waveguide rib 24 is again located closer to the planarsurface of the substrate 22 than the second optical waveguide rib 26.

In the first transition region 12, the cross section of the opticalwaveguide 10′ gradually varies so that the lateral light powerdistributions of propagating light modes evolve to become asschematically illustrated in FIG. 2A or 3A. In this example, thevariations involve lateral variations of both optical waveguide ribs 24,26. The variations include a smooth tapering that monotonicallydecreases the lateral width of the first optical waveguide rib 24 alongthe propagation direction. The inventor believes that the gradualdecrease of this lateral width causes more optical power of initial TEand TM propagating light modes to be re-distributed to the exterior ofthe first optical waveguide rib 24. The variations also include a smoothtapering that monotonically increases the lateral width of the secondoptical waveguide rib 26 along the propagation direction. The inventoralso believes that the gradual increase of this lateral width causesmore optical power of the initial TM propagating light mode to bere-distributed from the first optical waveguide rib 24 to the secondoptical waveguide rib 26. Without the tapered segments of the first andsecond optical waveguide ribs 24, 26, the inventor believes that lightinjected from the input optical waveguide would probably produce morescattering, reflection and excitation of higher modes, i.e., causinghigher insertion losses in the optical polarization rotator 10′.

In the polarization rotation region 14, the optical waveguide 10′undergoes smooth variations so that the propagating modes ofsubstantially linear polarization evolve so that their linearpolarizations are rotated, e.g., as schematically illustrated in FIGS.2A-2C and 3A-3C. Here, the variations involve a smooth decrease, alongthe propagation direction, in the lateral overlap between the crosssections of the first and second optical waveguide ribs 24, 26. Thisdecrease is due, at least in part, to an angular mis-alignment betweenthe axes of the first and second optical waveguide ribs 24, 26. In someembodiments, the decrease may also be due, in part, to an optionalmonotonic and smooth decrease in the lateral width of the second opticalwaveguide rib 26 along the propagation direction. Here, the variationsmay also involve a smooth and monotonic increase in the lateral width ofthe first optical waveguide rib 24 along the propagation direction.

In the second transition region 16, the cross section of the opticalwaveguide 10′ gradually varies so that the lateral light powerdistributions of the final propagating light modes will couple moreefficiently to the propagating light modes of the output opticalwaveguide 20. Here, the variation is a smooth tapering thatmonotonically increases the lateral width of the first optical waveguiderib 24 along the propagation direction. The second optical waveguide rib26 is either absent in the second transition region 16 or is offset by alarge lateral distance from the first optical waveguide rib 24, e.g., soas to not significantly influence the evolution of propagating lightmodes in the second transition region 16.

The above description describes the propagation direction of light asbeing from the left to right during operation of the opticalpolarization rotators 10, 10′ that are schematically illustrated inFIGS. 1A-1B and 4A-4C. Nevertheless, the optical polarization rotators10, 10′ also function when light propagates in the opposite directiontherein. That is, the optical polarization rotators 10, 10′ will alsorotate the linear polarization of light injected thereto from theoptical waveguide 20, which is shown at the right in FIGS. 1A-1B and4A-4C.

ILLUSTRATIVE EXAMPLE

FIGS. 4A-4C illustrate an example of the optical waveguide 10′ that canbe, e.g., fabricated from commercially available silicon-on-insulator(SOI) substrates and, e.g., using complementarymetal-oxide-semiconductor processes.

In the example, the first and second optical waveguide ribs 24, 26 andthe input and output optical waveguides 18, 20 are constructed asdescribed here. The first optical waveguide rib 24 is formed of aportion of the silicon layer of the initial SOI substrate and has athickness of about 200 nanometers (nm). The second optical waveguide rib26 is formed of silicon nitride and has a thickness of about 400 nm. Theinput and output optical waveguides 18, 20 are formed of portions of thesilicon layer of the initial SOI substrate and have thicknesses of about200 nm and lateral widths of about 500 nm to 600 nm.

In the example, the optical waveguide 10′ includes the optional spacerlayer 28, which is about 100 nm of silicon dioxide located between thefirst and second optical waveguide ribs 24, 26.

In the example, the first transition region 12, the polarizationrotation region 14, and second transition region 16 have lateral sizesand shapes as described below.

In the first transition region 12, the first and second opticalwaveguide ribs 24, 26 have laterally aligned central axes and extendover the full length of the region 12, which may be, e.g., about 50micro-meters (μm) long. The first optical waveguide rib 24 has aninitial lateral width of about 500 nm to 600 nm, at the boundary withthe input optical waveguide 18. Its initial lateral width, which aboutmatches that of the input optical waveguide 18, linearly decreases withdistance to a value of about 200 nm at or near the boundary of thepolarization rotation region 16. The second optical waveguide rib 26 hasan initial lateral width of about 100 nm or less, at or near theboundary with the input optical waveguide 18, and its lateral widthlinearly increases with distance to a value of about 200 nm at or nearthe boundary of the polarization rotation region 16.

In some embodiments, the lateral widths of one or both of the first andsecond optical waveguide ribs 24, 26, in the first transition region 12,reach a value of about 200 nm shortly before the boundary of thepolarization rotation region 14. For example, these lateral widths mayreach the 200 nm value at a distance of the order of 100 nm or more fromthe boundary and then, remain constant up to the boundary of thepolarization rotation region 14.

In the polarization rotation region 14, the first and second opticalwaveguide ribs 24, 26 extend about the full length of the region 14,which may be, e.g., between 250 μm to about 300 μm long. The firstoptical waveguide rib 24 has an initial lateral width of about 200 nm,at the boundary with the first transition region 12, and its lateralwidth increases linearly to about 260 nm, at the boundary with thesecond lateral region 16. The second optical waveguide rib 26 has alateral width of about 200 nm, at the boundary with the first transitionregion 12, and its lateral width may be constant or may decreaselinearly to a value about 150 nm or less, at the boundary with thesecond transition region 14. Due to a divergent lateral alignmentbetween central axes of the first and second optical waveguide ribs 24,26 in the polarization rotation region 14, the second optical waveguiderib 26 has a lateral offset of about 150 nm to about 200 nm from thecorresponding portion of the first optical waveguide rib 24, at and nearthe boundary with the second transition region 16.

In the second transition region 16, the first optical waveguide rib 24extends over the full length of the region 16, which may be, e.g., about50 μm. The first optical waveguide rib 24 has an initial lateral widthof about 260 nm, at the boundary with the polarization rotation region14, and its lateral width linearly increases with distance to a value ofabout 500 nm to 600 nm at the boundary with the output optical waveguide20.

The second optical waveguide rib 26 may or may not project into aninitial part of the second transition region 16. In cases where thesecond optical waveguide rib 26 projects into the second transitionregion 16, the lateral width of the second optical waveguide rib 26 maycontinue to smoothly decrease from about 150 nm to about 100 nm or less.In the second transition region 16, the gradual outward taper of thefirst optical waveguide rib 24 is insufficient to substantially decreasethe lateral offset between the first and second optical waveguide ribs24, 26 therein. Due to the large initial lateral offset, the secondoptical waveguide rib 26 does not cause substantial polarizationrotation of light while propagating in the second transition region 16.

In various embodiments of the polarization rotators 10, 10′ of FIGS.1A-1B and 4A-4C, differences between the material compositions of thefirst optical waveguide ribs 24, 24 and the material compositions of thesecond optical waveguide rib 26, 26 may be advantageous duringfabrication. In particular, the different material compositions mayenable use of the material of the first optical waveguide rib 24 and/orthe optional spacer layer 28 between the first optical waveguide rib 24and the second optical waveguide rib 26 as etch stops for anisotropicetches of the second optical waveguide rib 26 and/or the optional spacerlayer 28. Such different compositions can support simpler and/or moreaccurate etching fabrications of the optical waveguide ribs 24, 26 andthe spacer layer 28 if present.

FIG. 5 schematically illustrates an example method 30 for fabricating anoptical polarization rotator, e.g., the optical polarization rotator 10,10′ of FIGS. 1A-1B and/or 4A-4C.

The method 30 includes performing an etch of the top layer of thesubstrate, e.g., the silicon layer of the SOI substrate, to form thefirst optical waveguide rib 24 of FIGS. 4A-4C (step 32). The same etchmay be used to form additional silicon optical waveguides lateral to thefirst optical waveguide rib 24, e.g., to form the input and outputoptical waveguides 18, 20 of FIGS. 1A-1B and 4A-4C. The etch may use anyconventional process for mask-controlled etching silicon on such anexample SOI substrate, e.g., an anisotropic dry etch of silicon stoppingon silicon dioxide.

The method 30 may optionally include forming a layer of differentmaterial over the first optical waveguide rib 24 and, e.g., also overlaterally adjacent portions of the planar surface of the substrate 22and/or laterally adjacent optical waveguides (step 34). The layer ofdifferent material may be formed, e.g., by a conventional depositionprocess that forms silicon dioxide layers of optical quality.

If step 34 is performed, the method 30 may include chemical-mechanicalpolishing the layer of different material formed at step 34, to producea suitable spacer layer, e.g., the spacer layer 28 of FIGS. 1A-1B and4A-4C (step 36). The chemical-mechanical polishing, e.g., may produce aflat spacer layer of silicon dioxide, e.g., with a thickness of about100 nm or less on the first optical waveguide rib that was formed at thestep 32. The chemical-mechanical polishing may use any conventionalprocesses known by persons of ordinary skill in the art to be effectivefor polishing the layer of different material, which was formed at step34.

The method 30 also includes forming an optical layer over a portion ofthe first optical waveguide rib 24 and, e.g., over a portion of theplanar surface of the substrate 22 (step 38). The forming step 38 mayinvolve, e.g., depositing a layer of about 400 nm of silicon nitride onthe optional spacer layer and may use any conventional process, known topersons of ordinary skill, for depositing optical layers of siliconnitride. The optical layer, the first optical waveguide rib 24, and thespacer layer are typically formed of materials with different bulkrefractive indexes.

The method 30 also includes etching the optical layer to form the secondoptical waveguide rib 26, e.g., directly on the optional spacer layer 28or directly on the first optical waveguide rib 24 (step 40). The secondoptical waveguide rib 26 is formed such that first segments of the firstand second optical waveguide ribs 24, 26 form a stack verticallyoriented with respect to the planar surface of the substrate 22 and suchthat second segments of the optical waveguide ribs 24, 26 are relativelylaterally offset in a direction along the planar surface of thesubstrate 22. In embodiments where the optical layer is silicon nitride,the etch step 40 may use, e.g., any conventional process for etchingsilicon nitride. For example, the etch step may include an anisotropicdry etch for silicon nitride that substantially stops on the silicondioxide of the example spacer layer 28 and/or substantially stops onsilicon of the example silicon first optical waveguide rib 24 and/or thesilicon surface of the example substrate 22.

The method may also include depositing a cladding layer, e.g., a silicondioxide layer, over the waveguide structure produced at above step 40(step 42). For example, the cladding layer may have a thickness of a fewmicro-meters or more. The depositing step 42 may, e.g., use anyconventional process for depositing a thick optical cladding layer ofsilicon dioxide.

FIG. 6 schematically illustrates a method 50 for rotating thepolarization of a linear polarization component in a planar opticalpolarization rotator, e.g., the optical polarization rotators 10, 10′illustrated in FIGS. 1A-1B and 4A-4C.

The method 50 includes receiving, at a first end of the polarizationrotator, light from a planar waveguide, e.g., from the input opticalwaveguide 18 (step 52). The received light typically is in a linearpolarized mode, e.g., a TE or TM propagating light mode or an aboutin-phase combination of TE and TM propagating light modes.

The method 50 optionally includes propagating the received light througha first tapered waveguide segment of the optical polarization rotatorsuch that a substantial portion of the light power thereof is laterallyand/or vertically redistributed, e.g., in the first transition region 12of FIGS. 1A-1B and 4A-4C (step 54). The lateral and/or verticalredistribution of the light power is typically performed withoutsubstantial changes to the polarization of the received light.

The method 50 includes propagating the laterally redistributed lightpower along a hybrid optical waveguide segment of the polarizationrotator to rotate a linear polarization of the light by a substantialangle, e.g., an angle of greater than 45 degrees and often an angle ofabout 90 degrees, e.g., in the polarization rotation region 16 of FIGS.1A-1B and 4A-4C (step 56). The hybrid optical waveguide segment includesone segment in which a second optical waveguide rib is verticallylocated over a first optical waveguide rib, e.g., as in the left portionof the polarization rotation region 14 of FIGS. 1A-1B and 4A-4C,respectively. The hybrid optical waveguide segment includes anothersegment in which the second optical waveguide rib is substantiallylaterally offset from the corresponding segment of the first opticalwaveguide rib, because the two optical waveguide ribs are directed alongdivergent directions in the hybrid optical waveguide segment of thepolarization rotator. In the hybrid optical waveguide segment, the firstand second optical waveguide ribs are formed of materials havingdifferent bulk refractive indexes.

The method 50 may include propagating light from the hybrid opticalwaveguide segment through a final tapered waveguide segment of theoptical polarization rotator such that a substantial portion of thelight power is laterally and/or vertically redistributed, e.g., in thesecond transition region 16 of FIGS. 1A-1B and 4A-4C (step 58). Thisfinal lateral re-distribution of the light power may be performedwithout substantially changing the polarization of the light beingtransmitted. This lateral re-distribution produces a more efficientcoupling of the polarization rotated light to propagating light mode(s)of an output planar waveguide, e.g., output optical waveguide 20 ofFIGS. 1A-1B and 4A-4C.

FIG. 7 illustrates an optical modulator 43 that supports polarizationmode multiplexing of digital data. The optical modulator 43 includes a1×2 optical splitter 44, first and second optical waveguide arms 45, 46,and a 2×1 optical combiner 47.

The 1×2 optical splitter 43 receives substantially linearly polarizedlight from an optical source, e.g., TE or TM mode laser light, andseparates part of the received light into two light beams directed tothe two optical waveguide arms 45, 46. In FIG. 7, the optical source isillustrated as transmitting light of an example TE mode to the 1×2optical splitter 44.

Each optical waveguide arm 45, 46 connects a corresponding opticaloutput of the 1×2 optical splitter 43 to a corresponding optical inputof the 2×1 optical combiner 47. The 1×2 optical splitter 43 transmitslight of about the same linear polarization, i.e., illustrated asexample TE propagating light mode, to the two optical waveguide arms 45,46. Each optical waveguide arm 45, 46 includes an electro-opticalmodulator 48_1, 48_2, which is electrically connected to receive astream of electrical digital data signals, i.e., DATA_1 or DATA_2. Eachelectro-optical modulator 48_1, 48_2 modulates the phase and/oramplitude of light propagating there through to carry the received steamof received digital data signals, i.e., DATA_1 or DATA_2. The secondoptical waveguide arm 46 also includes an optical polarization rotator49, e.g., the optical polarization rotator 10, 10′ of FIGS. 1A-1B or4A-4C, which rotates the linear polarization of light therein. Thepolarization rotator 49 rotates the linear polarization by, e.g., atleast, 45 degrees, and more preferably by about 90 degrees so thatmodulated light beams from the two optical waveguide arms 45, 46 willhave approximately orthogonal polarizations when recombined in the 2×1optical combiner 47. The modulated light beams from the two opticalwaveguide arms 45, 46 are combined in the 2×1 optical combiner 47 topolarization-multiplex the two data-modulated optical carriers.

In some other embodiments, the order of the optical polarization rotator49 and the electro-optical modulator 48_2 may be inverted on the opticalwaveguide arm 46.

The optical modulator 43 may be fabricated as a fully opticallyintegrated device or as a partially optically integrated device.

FIG. 8 illustrates a portion of an optical receiver 60, which isconfigured to demodulate data from an optical carrier, wherein theoptical carrier is polarization multiplexed with data streams DATA_1 andDATA_2. The optical receiver 60 includes a first optical polarizationbeam splitter 62, an optical polarization rotator 64, and first andsecond optical demodulators 66, 68. The optical polarization rotator 64is connected to receive the optical signal from one optical output ofthe optical polarization beam splitter 62 and to optically rotate thelinear polarization of said received optical signal. Due to the actionof the optical polarization rotator 64, both optical demodulatorsreceive light in a substantially similar or the same linearpolarization, e.g., illustrated as an example TE propagating light modein FIG. 8. Thus, while each optical demodulator 66, 68 demodulates adigital data, e.g., data streams DATA_1′ and DATA_2′, from a differentlinear polarization component of the light received at the input of theoptical receiver 60, the two optical demodulators 66, 68 process lightbeams of substantially similar or the same linear polarization.

In some embodiments, the optical output of the optical polarizationrotator 64 may connect directly to an optional second opticalpolarization beam splitter (not shown) and one of the optical outputs ofthe second optical polarization beam splitter then connects directly toone optical input of the second optical demodulator 68. In suchembodiments, the second optical polarization beam splitter functions asa clean-up polarization filter for the optical polarization rotator 64.

In the optical receiver 60, the intermediate conversion of the linearpolarization of one light beam may be useful to ensure that theprocessing of each data carrying optical carrier is substantiallysimilar. That is, the intermediate conversion can be used to avoidissues related to polarization dependencies of the opticalcharacteristics of media and waveguides of the optical demodulators 66,68.

The Detailed Description of the Illustrative Embodiments and drawingsmerely illustrate the principles of the invention. It will thus beappreciated that those skilled in the art will be able to devise variousarrangements that, although not explicitly described or shown herein,embody the principles of the inventions and are included within theclaimed inventions. Furthermore, all examples recited herein areprincipally intended to be only for pedagogical purposes to aid inunderstanding the principles of the inventions and concepts contributedby the inventor to furthering the art, and are to be construed as beingwithout limitation to such specifically recited examples and conditions.Moreover, all statements herein reciting principles, aspects, andembodiments of the inventions, as well as specific examples thereof, areintended to encompass equivalents thereof.

1. An apparatus, comprising: an optical source; and an opticalpolarization rotator being configured to receive light from the opticalsource, the optical polarization rotator including first and secondoptical waveguide ribs located along a planar surface of a substrate,the second optical waveguide rib being located farther from the surfacethan the first optical waveguide rib; and wherein first segments of theoptical waveguide ribs form a vertical stack over the substrate andsecond segments of the optical waveguide ribs are offset laterally in adirection along the surface; and wherein the first and second opticalwaveguide ribs are formed of materials with different bulk refractiveindexes.
 2. The apparatus of claim 1, further comprising a spacer layerbeing located between the first and second optical waveguide ribs andbeing formed of a material with a different bulk refractive index thanthe materials of the first and second optical waveguide ribs.
 3. Theapparatus of claim 2, wherein the spacer layer is vertically thinnerthan the first optical waveguide rib.
 4. The apparatus of claim 1,wherein the first optical waveguide rib is formed of a same material asa portion of the planar surface of the substrate.
 5. The apparatus ofclaim 1, wherein one of the optical waveguide ribs is formed of asemiconductor and the other of the optical waveguide ribs is formed of adielectric.
 6. The apparatus of claim 1, wherein the opticalpolarization rotator is configured to rotate a polarization of receivedlinearly polarized light by at least 45 degrees.
 7. The apparatus ofclaim 1, further comprising a transition region in which the firstoptical waveguide rib has a width lateral to the surface thatmonotonically tapers from a larger value at an input planar opticalwaveguide to a smaller value at the first segment of first opticalwaveguide rib.
 8. The apparatus of claim 7, wherein the second opticalwaveguide rib has a width lateral to the surface that monotonicallytapers from a smaller value at an end thereof nearer to the input planaroptical waveguide to a larger value at the first segment of the secondoptical waveguide rib. 9-10. (canceled)
 11. The apparatus of claim 1,further comprising an optical modulator having a 1×2 optical splitter,first and second optical waveguide arms, and a 2×1 optical combiner,each optical waveguide arm connecting a corresponding optical output ofthe 1×2 optical splitter to a corresponding optical input of the 2×1optical combiner; and wherein each optical waveguide arm includes anelectro-optical modulator capable of modulating the phase and/oramplitude of light propagating there through responsive to electricalsignals received therein; and wherein an optical polarization rotator islocated in one of the optical waveguide arms. 12-20. (canceled)
 21. Theapparatus of claim 1, wherein optical polarization rotator is configuredto receive laser light from the optical source.
 22. The apparatus ofclaim 1, wherein optical polarization rotator is configured to receivelinearly polarized light from the optical source
 23. An opticalreceiver, comprising: a polarization beam splitter having first andsecond optical outputs; and an optical polarization rotator includingfirst and second optical waveguide ribs located along a planar surfaceof a substrate, the second optical waveguide rib being located fartherfrom the surface than the first optical waveguide rib; and wherein firstsegments of the optical waveguide ribs form a vertical stack over thesubstrate and second segments of the optical waveguide ribs are offsetlaterally in a direction along the surface; wherein the first and secondoptical waveguide ribs are formed of materials with different bulkrefractive indexes; and wherein the first optical output of thepolarization beam splitter is connected to transmit light to the firstoptical waveguide rib and the second optical output of the polarizationbeam splitter is connected to transmit light to a third optical output.24. The optical receiver of claim 23, wherein the optical polarizationrotator is connect to transmit light to a fourth optical output; andwherein the optical receiver is configured to transmit light ofsubstantially the same linear polarization to the third and fourthoptical outputs.
 25. The optical receiver of claim 24, furthercomprising: a first optical demodulator connected to receive light fromthe third optical output; and a second optical demodulator connected toreceive light from the further optical output.